Secondary flow effects are responsible for up to a third of the overall losses in a gas turbine. Therefore, understanding these flow structures and the parameters which control them is important for the design of turbomachines. One main trend during the last decades in gas turbine design for flight propulsion is to target higher compactness, so as to reduce the overall engine size. Reduced compressor and turbine sizes, however, yield smaller aspect ratios of the blades with increased influence of the endwalls. The study and characterisation of these flow structures, whose main feature is the Tip Leakage Vortex (TLV) inside the tip gap, is difficult, mainly due to the complex geometry in rotating machines with complex 3D flow. Due to this complexity, historically, rotating blade rows were frequently reduced to linear cascades. In this reduction, however, several characteristics are lost, such as Coriolis effects and centrifugal forces, and the impact of  relative motion of the endwalls, for example. The influence of these parameters on the secondary flow,  however, is important to correctly transfer performance estimations from linear cascades to rotating blade rows. The present contribution focusses on the effect of imposing a static endwall instead of a moving one.

To investigate this issue, Direct Numerical Simulations (DNS) of the flow through a linear compressor cascade are presented, juxtaposing results obtained with different relative speed between endwall and compressor blades. Experimental data from wind tunnel measurements are available for validation. The Reynolds number of the study was reduced to make DNS feasible, which provided reliable results for vortex structures and transition issues.

Induced by the endwall relative motion, the flow pattern is significantly altered with respect to the non-moving case. Time averaged data show a displacement of the TLV towards the middle of the passage, which additionally depends on the magnitude of the endwall speed. Furthermore, the TLV remains closer to the endwall, compared to the case without relative motion. As a result, since the TLV is located towards the middle of the passage and nearer to the endwall, a blockage characterised by an axial velocity deficit is observed in the passage flow. The shift of the TLV towards the middle of the passage has three further consequences. First, flow exiting the gap towards the trailing edge does not interact significantly with the TLV, so that new and smaller flow structures are created by the flow passing towards the trailing edge. Second, the pressure side of the neighbouring blade becomes affected by the TLV. In particular, pressure and skin friction coefficients near the blade tip at the pressure side show increased values for cases with endwall motion compared to the case without relative endwall motion. Lastly, endwall motion and resulting TLV shift have an impact on the flow structure at the exit of the blade passage and the corresponding pressure losses. In addition to the statistical data in terms of mean flow and fluctuations spectral analysis of the flow was performed revealing a marked difference between cases with and without endwall speed. Clear coherent flow structures with a pronounced frequency are observed in the former case which is not found without endwall speed. Their origin and dependence on the wall speed are analysed.


Study of Relative Endwall Motion Effects in a Compressor Cascade Through Direct Numerical Simulations

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.